Receiver automatic volume control switch



2 Sheets-Sheet l A/VZ/AL V010 K. W. JARVI$ RECEIVER AUTOMATIC VOLUMECONTROL SWITCH Filed Sept. 16, 1935 12/41 Pam/0 (/7 WA UM @efi. 25:3,193 'Z.

\i Q Q aw KENNETH W. JARWS w fw b ATTORNEY /0 100 /000 70,000 /00,000INVENTOR 26,1 1937. K. w. JARVIS zmm RECEIVER AUTOMATIC VOLUME CONTROLSWITCH Filed Sept. 16, 1935 2 Sheets-Sheet 2 (UNA 070; 70

INVENTOR KENNETH W. JARVIS BY .QM

ATTORNEY Patented Oct. 26, 1937 umrso sm'rss PATENT OFlQE Kenneth W.Jarvis, New Rochelle, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Application September 16, 1935, Serial No.40,685

1 Claim.

My present invention relates to gain control arrangements for radioreceivers, and more particularly to a radio receiving system providedwith a switch for rendering the receiver automatic gain control networkinefficient during periods of tuning in the desired station. 7 In normalnon-automatic volume controlled receivers, it is the practice to tune tothe desired station by tuning to the point of the loudest signal. As theselectivity characteristic of the receiver is generally quite sharp,this point of maximum volume is found quite easily. It is well known inthe prior art that for maximum discrimination against undesiredstations, this tuning to the peak of the selectivity characteristics, orfrequency response characteristic, is quite necessary. The fidelity andquality of the received signals are also best when the receiver is tunedto this proper point.

As is well known automatic volume control is utilized in receivers ofmodern construction, in most cases, to eliminate the effects of signalfading. In such receivers the amplitude of the acoustic volume ismaintained practically constant by automatically increasing the receivergain whenever the incoming signal strength decreases. The control pointfor such automatic control is generally at, or near, the detectorproducing the final audio frequency signal. For this reason any changeahead of this point, such as signal fading, is compensated for to theextent of the automatic volume control.

Mistuning also normally reduces the signal intensity at the detector andthe control point, and so brings into operation the automatic control,thus raising the acoustic level to its normal output; and this occurseven though the receiver is mistuned. Hence, with automatic volumecontrol tuning to the correct point through apparent signal intensity isnot possible.

It may, therefore, be stated that it is one of the principal objects ofthe present invention to enable such correct tuning to be obtainedWithout sacrificing any ofthe advantages of automatic volume control.

Another important object of the present invention may be said to residein the inclusion in a radio receiver of the type utilizing automaticvolume control, of a manually actuatable switch mechanism for renderinginefiicient the automatic volume control mechanism upon actuation of thetuning means of the receiver.

Still other objects of the invention may be said to reside in thegeneral improvement of receivers utilizing automatic volume control sothat they receiver.

operate in a manner rendering tuning simple, and more especially toprovide such improvement in an economical and efficient manner.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawings:

Fig. 1 graphically shows the operation of a receiver which does notembody the present invention,

Fig. 2 graphically illustrates the operation of a receiver employingautomatic volume control,

Fig. 3 is a circuit diagram of a receiving system utilizing the presentinvention,

Fig. 4 is a voltage-current curve of the automatic volume control tubeshown in Fig. 3,

Fig. 5 graphically indicates the magnitude of the volume control biaswith respect to the signal producing it,

Fig. 6 shows a mechanical embodiment of the automatic volume controlswitch employed according to the present invention.

Referring now to the accompanying drawings, wherein like referencecharacters in the different figures indicate similar elements, in Fig. 1there is shown in graphic manner the variation of acoustic output withdial position for the two methods of volume control used in a radio Thefull line curve I indicates the sharply selective condition of thereceiver, and it will be seen that the acoustic output varies almost inproportion as the dial is rotated. The full line curve I represents theacoustic response when the receiving system does not utilize automaticvolume control (hereinafter referred to .as AVC) and a manual volumecontrol device is used. The obvious point for correct tuning is at thepoint 2, quite easily determined with the given shape of curve I. Thebroken line curve 3 represents the acoustic response when the receiveremploys automatic volume. control. It will be seen that in tuning awayfrom the correct operating point 2 the receiver gain decreases.

However, the automatic volume control action tends to restore this gainby reducing the negative bias on the gain control electrodes of theSignal amplifier tubes. As a result the acoustic output does not fall,but stays substantially conmanner represented by the broken line 6.

stant over a widerange of dial movement, as shown. The correct turningpoint 2 is almost impossible to find as a result of this action of theAVG mechanism.

In Fig. 2 there. is shown the relation between acoustic outpu and signalstrength in microvolts. The full line curve 45 shows the response withrespect to the signal input. The signal input increasing from zero, theoutput rises as shown. At the point 5 the AVG action begins, and thecurve flattens off. Without this AVC action the output would probablyrise in the In the case of detuning the gain due to the frequencycharacteristic of the resonant circuits falls. The output would fallexcept that the AVG action compensates. some point on the right handside indicates the correct tuning point, while detuning corresponds tosliding back along the curve to the left; of course, eventually losingthe signal despite the AVG action. At first the detuning causes verylittle loss in signal; it will be observed that the analysis in Zconfirms and explains that of Fig. 1.

Fig. 3 shows a portion of a radio receiving system embodying one form ofthe present invention. It will be understood that only such portions ofthe radio receiving system are shown as are necessary for a properunderstanding of this invention. The numeral 7 indicates a tunedtransformer feeding an amplifier tube 8. The primary of the transformeri may be coupled to one or more stages of radio frequency signalamplification, and it is to be understood that the receiver may be ofthe superheterodyne type, or of the well known tuned radio frequencytype. For example, if the receiver is of the superheterodyne type, thenthe tube 3 may be one of the I. F. amplifier tubes, and the transformer1 will then bepreceded by the usual radio frequency amplifier and firstdetector stages. If the receiver is of the tuned radio frequency type,then each of the tunable circuits of the receiver will include variablecondensers arranged in the well known uni-controlled manner.

The amplifier 8 feeds through the tuned transformer 9 a detector IQ. Thenumeral it designates a source of positive potential which is includedin the plate circuit of amplifier tube 8, and the input circuits oftubes 8 and it each include variable tuning condensers. Of course, ifthe receiver is of the superheterodyne type the condensers in the inputcircuits of tubes 8 and ID will be fixed, and in that case the presentinvention is utilized in connection with the variable tuning condenserscommonly employed ahead of the first detector tube. The detector tube IDdemodulates the amplified radio frequency signal, and the demodulatedsignal is transmitted to the subsequent audio amplifying network, andfinal reproducer, through the transformer Ii. The detector tube ill isshown as of the self-biased type, and the cathode lead of the detectortube includes the usual self-bias resistor l2 which is shunted by theby-pass condenser !3. The numeral it represents a source -of positivepotential for the plate of detector tube Ill.

The AVC network includes an electrondischarge tube l6 which has its gridI"! connected by way of condenser E5 to the signal grid of detector tubeIt. The plate N3 of the AVG tube I6 is connected to a source of positivepotential through the resistor Id. The cathode 2c of the AVG tube is thelow potential point. so far as circuited.

signal currents are concerned, and is by-passed by the condenser 2!.Resistors 22, 23 and 24 are connected in series across the source ofpotential 25. A switch 26, having two contacts 21 and 28, is provided,the fixed end of the switch being connected to the junction of resistorsl9 and 29.

Contact 2? is connected to the common junction point 36 of resistors 29,23 and 24. Contact 28 connects to the sliding contact 30, the latterbeing slidable over resistor 24. The slidable tap 30 is connected bylead 30 to the cathode 20 of tube l6.

When the switch 26 is connected to contact 21, the resistor 29 which hasa low value, is short- The positive potential between the points 20 and36 across the resistance 24 is applied to the plate !8 through theresistance 18. The bias potential for tube I5 is the voltage between thepoints 24' and 3'! on resistor 24. The slidable tap 39 is adjusted togive a grid bias, as applied to the grid l1 through leak resistor 33,which is several volts below cut-off.

With no incoming signal, no current flows through the resistor I9, andthe grid potentials;

of the amplifier tube 8 is the initial bias provided by the voltage dropacross the resistor 23. One side of resistor 23 is grounded, as is thecathode of tube 8, and the other side of the resistor is connectedthrough the resistors i8, 34 and the secondary of transformer l to thesignal grid of controlled amplifier 8.

Now, if an incoming signal reaches the tube H] with an intensity greaterthan several volts, this signal transferred to the AVG tube I 6 by thecondenser l5 will cause current to flow in the plate circuit of tube l6and through the resistor IS. The voltage developed across the resistorl9 biases the amplifier 8 to a higher value than normal, and so reducesthe gain with the result. that a control action is developed which tendsto prevent further increase in signal.

This same bias voltage may be similarly applied to other of the signaltransmission tubes of the receiving system, and preceding tube 8, as,

is indicated by the AVG lead 35. Plate 18 of the AVC tube is by-passedby the condenser 32 to remove undesired alternating current components,and to improve its rectifying action. Initial adjustment of the point24' serves to regu-, late the signal strength at which current begins toflow into the plate circuit of tube 16. After this AVC action begins,little further increase in acoustic output is possible, and consequentlythe adjustment of the sliding tap 30 determines the volume output of theradio receiver. This volume is then automatically maintained by theabove described action.

is connected in shunt between the points 24 and 3B whenever the switch26 connects to contact 28. This lowers the equivalent resistance betweenpoints 24 and 36. By properly choosing the magnitude of the resistor 29,the resulting bias voltage can be made such a value as to cause littlechange in the acoustic output, when the switch 25 is moved from the AVGposition to the manual control position. Again, with resistor 29connected between switch points 26 and 21, the switch, and consequentlythe grid circuit is never completely open. When the switch 26 is in themanual volume control position, movement of tap 30 will change the biasvoltage, and hence the volume of the receiver.

Fig. 4 shows a curve 38 which gives the plate current of tube 16 throughresistor l9 as a function of grid bias. In other words this curve showsthe voltage between the points 24 and 31. Various possible operatingpoints 39 to 43 inclusive are indicated. The point 39 is above cut-01fthe point 40 is at cut-off; and the other points 4| to 43 inclusive arebelow cut-off.

Fig. shows a series of curves each marked to correspond with theoperating points indicated in Fig. 4. The horizontal dashed line 44 isthe cut-off voltage of the amplifier system. If that voltage wereobtained from the AVG tube, the amplifier 8 and previous amplifierswould not amplify and no signal would result. As the AVG bias is causedby signal coming through the amplifiers, this extreme condition couldnot be reached. However, the relative maximum amplitudes approximate therelationship of the intersections of the curves 39 to 43 with the line44. Thus, the greater the negative bias on the grid ll of the AVG tube,the greater will be the average acoustic output. Therefore, movement ofthe point 24 on the resistor 24 will regulate the average volume whenthe switch 26 is connected to contact 21.

Moving the slider 30 towards the upper end 36 of resistor 24 increasesthe volume in both the manual and AVC positions. Moving the slider 30towards the lower point 31 lowers the volume in both positions. This isnot a mere coincidence, but obviously a necessary condition forsatisfactory operation of the presently disclosed dual control system.In Fig. 6 there is shown a simple and efiicient manner of producing theswitching action described in connection with Fig. 3, in an automaticmanner. In Fig. 6 the numeral 45 denotes the variable tuning condensersof the receiving system, and the rotors of the variable condensers aremounted on the shaft 46, the shaft carrying the usual tuning dial 4'1.The shaft 46 may be rotated by engaging the clutch 48 by motion of theshaft 49 towards the condenser 45.

The shaft 49 is constructed to be free to turn in the insulating block50, and the latter is normally pulled in clutch-opening position by thesprings 5| and 52. The bearing 53 is provided, and also acts as a stopfor the retaining ring 54. The tuning knob 55 is secured to the shaft49.

To tune the variable condenser it is necessary to push in the controlknob 55 thus engaging the clutch 48. The condenser 45 can then berotated to any desired position. In so pushing in the knob 55 theinsulating block 50 moves forward and operates the switch 56. Thecontacts of this switch correspond to the points 26, 21 and 28 of thecircuit shown in Fig. 3. It will be observed that whenever the condensercan be tuned, the volume control is equivalent to the normal manualcontrol, and consequently the tuning characteristic will be like that ofcurve I in Fig. 1.

Whenever the receiver is being used, but not during the tuning process,the AVG connection is utilized, and so gives its benefit to theoperation of the receiver. It will, therefore, be seen that there isthus provided simple and efiicient means for tuning in signals in asharp manner, while providing the advantages of AVG action during thelonger and more important listening period.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaim.

What I claim is:

In a receiver of the type including an adjustable signal selector, asignal transmission tube, a demodulator, and an automatic gain controltube having control bias connections to said transmission tube, a sourceof direct current potential connected by electrical connections,including said control bias connections, to the transmission tube toprovide normal operating bias therefor, connections from the source tothe control tube to energize the latter, and means. responsivetoactuation of the selector, for rendering the connections to thecontrol tube ineffective to energize the latter without interferencewith said normal bias connections.

KENNETH W. JARVIS.

